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Capturing Durability of High Recycled Binder Ratio (RBR) Mixtures

Description:

Environmental sustainability and economic benefits motivate maximum use of recycled asphalt materials, including reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), in asphalt mixtures. However, adequate cracking performance initially and with time when these aged materials are added to virgin asphalt-aggregate mixtures must be maintained. To preclude cracking, state departments of transportation (DOTs) limit the RAP and RAS contents and overall recycled binder ratio (RBR) from these materials.

NCHRP 9-58 The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios produced a draft AASHTO Standard Practice for high 0.3-0.5 RBR binder blends and mixtures that includes:

(1) component materials selection guidelines for virgin binders, recycling agents, and recycled materials and their proportions;

(2) a recycling agent dose selection method;

(3) a recycled binder availability factor for mix design; and

(4) binder blend rheological evaluation tools

(5) mixture performance evaluation tools

The evaluation tools include aging protocols, recycling agent blending methods, and laboratory tests and corresponding thresholds for adequate rutting and cracking performance.

This draft AASHTO Standard Practice was developed based on five field projects in TX, NV, IN, WI, and DE; each with multiple test sections but none with recycling agents at higher dosages selected by the method developed to maintain rutting resistance while maximizing cracking resistance with aging based on laboratory tests. The guidelines and tools in the draft AASHTO Standard Practice allow for evaluation of rutting and cracking resistance for a specific combination of materials, but moisture susceptibility was not addressed in detail but is of critical concern with rejuvenation by recycling agents. Aging protocols associated with the evaluation tools also likely need to be updated based on the results of NCHRP 9-54 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction and NCHRP 9-61 Short- and Long-Term Binder Aging Methods to Accurately Reflect Aging in Asphalt Mixtures and the continued challenge of practically capturing field aging in different climates in the laboratory, especially for modified binder blends and mixtures. In addition, further verification of the cracking performance thresholds in the draft AASHTO Standard Practice can be realized by continued monitoring of the five field projects from NCHRP 9-58.

Objective:

The objectives of this research are to expand and update the draft AASHTO Standard Practice produced in NCHRP 9-58 for high RBR binder blends and mixtures to capture durability with: · tools to evaluate moisture susceptibility · updated aging protocols · verified cracking performance thresholds

Benefits:

An economic analysis conducted as part of NCHRP 9-58 indicates that doubling RAP contents from 20 to 40% with the use of recycling agents at dosages up to 8-10% can result in savings from $6 to $8 per ton HMA, or from $0.30 to $0.40 per one percent RAP, or 7-10% of in-place HMA price when virgin materials costs are relatively high. In addition to economic benefits, NAPA estimates that almost 50 million cubic yards of landfill space was saved by recycling RAP and more than $2.2 billion was saved in 2017 by using RAP and RAS instead of virgin materials. These substantial environmental sustainability and economic benefits can be realized with a comprehensive draft AASHTO Standard Practice for high RBR binder blends and mixtures that includes durability guidelines and evaluation tools that consider moisture susceptibility and aging with rutting and cracking performance.

Related Research:

This research complements that done in NCHRP 9-58 but expands the resulting draft AASHTO Standard Practice for high RBR binder blends and mixtures to include durability guidelines and evaluation tools that align with aging protocols developed in NCHRP 9-54, NCHRP 9-52 Short-Term Laboratory Conditioning of Asphalt Mixtures and the associated NCHRP 9-52A, and NCHRP 9-61. Consideration should also be given to: (1) the moisture susceptibility evaluation guidelines developed for warm mix asphalt (WMA) mixtures in NCHRP 9-49 Performance of WMA Technologies: Stage I – Moisture Susceptibility and validated in the associated NCHRP 9-49B and (2) the recommendations for cracking tests and thresholds from NCHRP 9-57 _Experimental Design for Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures _and the ongoing follow-on NCHRP 9-57A. Without the specific inclusion of the durability issues of moisture and oxidative aging, only the environmental benefits are attained through the increased use of recycled materials and the engineering performance and therefore economic benefits are unrealized.

Tasks:

1. Evaluate current and relevant tests and methods for characterizing moisture susceptibility of asphalt mixtures including the guidelines developed for WMA in NCHRP 9-49 and validated in the follow-on NCHRP 9-49B.

2. Review aging protocols developed in NCHRP 9-54, NCHRP 9-52/9-52A, and NCHRP 9-61 and binder aging modeling results from NCHRP 9-58.

3. Define practical parameters to capture field aging of high RBR mixtures in the laboratory that include initial quality and rate of aging.

4. Further verify cracking performance thresholds developed in NCHRP 9-58 by monitoring five established field projects and evaluate alternate cracking tests and thresholds recommended in NCHRP 9-57/57-A.

5. Expand and update the draft AASHTO Standard Practice for high RBR binder blends and mixtures produced in NCHRP 9-58 to include durability in terms of moisture susceptibility and aging.

Implementation:

The results of this research will be used by highway agencies, asphalt mixture designers, and asphalt paving contractors to produce durable asphalt mixtures for pavements with high RBR that are environmentally sustainable and economically viable. Implementation activities may include: · review of the expanded and updated draft AASHTO Standard Practice for high RBR binder blends and mixtures by the FHWA Mixture Expert Task Group (ETG) for comment and identification of champions for national implementation · a national workshop with invited participants for briefing on the expanded and updated draft AASHTO Standard Practice for high RBR binder blends and mixtures

Relevance:

This research will result in a comprehensive draft AASHTO Standard Practice for high RBR binder blends and mixtures that includes durability guidelines and evaluation tools for moisture susceptibility and aging that can be used by highway agencies, asphalt mixture designers, and asphalt paving contractors to identify materials combinations (virgin binder, recycled materials including RAP and RAS, recycling agent, and virgin aggregate) where significant environmental sustainability, economic, and engineering performance benefits can be realized simultaneously.

Sponsoring Committee:AFK50, Structural Requirements of Asphalt Mixtures
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Amy Epps Martin, Matt Corrigan, Shane Underwood
Source Info:Reports and published papers and presentations from NCHRP 9-58, NCHRP 9-54, NCHRP 9-52, NCHRP 9-52A, NCHRP 9-61, NCHRP 9-57, NCHRP 9-57A, NCHRP 9-49, and NCHRP 9-49B
Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage 2017, IS 138, National Asphalt Pavement Association
Date Posted:02/19/2019
Date Modified:02/26/2019
Index Terms:Durability, Recycled materials, Bituminous binders, Asphalt mixtures, Asphalt pavements,
Cosponsoring Committees: 
Subjects    
Highways
Materials
Pavements

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